The present invention relates generally to fixture of a board, and more particularly to fixture of a package board having a heat radiator mechanism. The present invention is suitable, for example, for fixture of a beat sink and a Land Grid Array (“LGA”) package. The present invention is also relates to a printed circuit board, such as a motherboard, mounted with such a package board, and an electronic apparatus, such as a server, equipped with the printed circuit board.
The recent development of electronic apparatuses has increasingly demanded supply of sophisticated and inexpensive electronic apparatuses. LGA packages have been conventionally proposed for this demand. The LGA package is a type of a package board that uses an LGA socket, instead of soldering, for connection with a printed circuit board (also referred to as a system board or motherboard). Prospective demands for L(3A packages, are expected due to their shorter assembly time and lower cost than Ball Grid Array (“BGA”) packages that use soldering for the printed circuit board, since the LGA package includes no soldering step to the printed circuit board, which would otherwise cause thermal breakages of an electronic circuit.
In general, the LGA package is mounted with an IC and an LSI, which serve as a CPU, and the calorific value increases as the CPU improves its performance. Accordingly, a cooling device called a heat sink is thermally connected to the CPU through a heat spreader. The beat sink includes cooling fins adjacent to the CPU, which radiate the heat from the CPU through natural cooling. The conventional LGA package is connected to the printed board through the LGA socket, and clamped by the heat sink.
A description will be given of the conventional fixture of a heat-sink-cum LGA package. Here, FIG. 9 is a schematic sectional view for explaining the conventional fixture of the heat-sink-cum LGA package. As illustrated, a ceramic package board 10 mounted with air LSI 2 via bumps 4 and underfill 6 is mounted onto a printed circuit board 14 via an LGA socket 12, and connected thermally to a beat sink. 18 via a heat spreader 16 The LSI 2 and heat spreader 16 are adhered to each other through adhesives 3. The LGA socket 12 has an LGA terminal section 12a as actual connection part. Pressure member 20 is fixed onto a clamp plate 22 provided on a bottom surface of the printed circuit board 14, through perforations in the heat sink is and the printed circuit board 14. The pressure members 20 compress the heat sink 18 and consequently the heat spreader 16 entirely.
The conventional LGA package thus mounts the LSI 2 onto the ceramic package board 10 so as to prevent the LSI 2 and the board 10 from bending in mounting the LSI 2 because the LSI 2 and ceramic have close coefficients of thermal expansion. Although the underfill 6 directly contacts the package board 10, the underfill 6 has so thin that a difference in coefficient of thermal expansion between the LSI 2 and the package board 10 is prevailing. Use of a material that has a coefficient of thermal expansion close to that of the LSI 2 for the heat spreader 16 to be attached to the rear surface of the LSI 2 would enable the heat spreader 16 to maintain its flatness. Good flatness would reduce influence of pressure from the heat spreader 16 surface on a connection part between the LSI 2 and the package board 10, and a connection part between the LSI 2 and the beat spreader 4. High rigidity of the package board 10 enables the package board 10 to endure an application of the pressure from the top.
In general, the LGA socket 12 makes a terminal section 12a of gummy resin or elastomer blended, with silver flakes at a predetermined density. Silver flakes contact and maintain :good conductivity when this terminal is pressurized. As the applied pressure increases, the contact density of silver flakes increases but the resistance also increases resultantly. It is therefore recommended to adjust applied pressure to target pressure in mounting the LGA package onto the system board for good conductivity without increasing the resistance.
As the above conventional structure has used the package board 10 and the LSI 2 having approximately equal coefficients of thermal expansion, the stress generated with thermal expansion and shrinkage between them becomes very small. No substantial stress causes a breakage between the package board 10 and LSI 2 even when the above pressure is applied due to the similar thermal expansions.
As the necessary pressure to fix the heat sink 18 for the LSI 2 may be smaller than the pressure to ;the terminal section 12a of the LGA socket 12, the pressure applied to the terminal section 12a of the LGA socket 12 also fixes the heat sink 18.
Other prior art disclose, for example, a semiconductor device that includes a printed circuit board, a semiconductor element, a stiffener, and a metal plate, as in Japanese Patent Application Publication No. 11-284097, paragraph Nos. 0024 to 0035, and FIGS. 1 and 2, and a clamp mechanism for applying uniform pressure among a heat sink, an IC, and a socket, as in Japanese Patent Application Publication No. 2001-60778, paragraph Nos. 0006 to 0009, and FIGS. 1A, 1B, 2A and 2B.
Instant inventors have considered use of resin for the package board 10 instead of ceramic for lower cost and easier processing. A resin board is thinner than a ceramic board, and thus expected to exhibit a more improved electric characteristic than the ceramic board.
While the above predetermined pressure is necessary for good conductivity of the LGA terminal section 12a, this pressure causes the stress between the resin package board and the LSI because they have different coefficients of thermal expansion. In other words, as shown in FIG. 9, when the pressure necessary to connect the LGA socket 12 and the printed circuit board 14 to each other is applied between the heat sink 18 and the package board 10, the large force is applied to a connection part between the LSI 2 and the heat spreader 16, and a connection part between the LSI and the resin board, causing cracks in these connection parts. In particular, a coefficient of thermal expansion of the resin board is so different from that of the LSI 2 that the LSI 2 is caused to bend, and the heat spreader 16 adhered to the rear surface of the LSI 2 also bends, as shown in FIG. 10, under influence by bending connection part between the package board 10A and the LSI 2. The large pressure applied to correct this camber is excessive load for the connection part. Here, FIG. 10 is a schematic sectional view for explaining the conventional problem.